This invention relates to down-hole bearing assemblies, for example for maintaining a survey instrument assembly at a fixed angular orientation with respect to a core barrel supported by the bottomhole assembly of a rotating drill string and rotationally isolated therefrom.
It is a common practice when drilling for oil to take a core sample of the formations being drilled and to subsequently analyze the sample in the laboratory with a view to obtaining information on the geology of the formations, particularly with a view to obtaining information on the quantity and distribution of formation hydrocarbons.
In one method of coring an assembly comprising an electronic multi-shot measurement instrument, a core barrel and a coring bit is attached to the end of the bottomhole assembly and the complete assembly is introduced into the borehole. The coring bit is then rotated by the drill string so as to cut a core sample which is taken up by the core barrel. After withdrawal of the assembly from the borehole, the core barrel containing the core sample is detached from the bottomhole assembly. In this method it is required that the core barrel and the survey instrument should be maintained at the same relative angular orientation and such that both are rotationally isolated from the drill string so that the orientation of the core sample when cut from the surrounding formations will be known. However, this is difficult to achieve since the drill string supporting the survey instrument and core barrel will rotate, and conventional attempts to rotationally isolate the survey instrument from the drill string have not proved satisfactory in practice.
It is an object of the invention to provide a down-hole bearing assembly which is capable of effectively maintaining the survey instrument assembly at a fixed angular orientation with respect to the core barrel during core cutting.